FIG. 14 shows such a forging or cold-pressing tool already known (see JP-B-62342/1990 and corresponding U.S. Pat. No. 4,417,464). The tool is a die comprising a nib 2 formed by six nib pieces 3 isosceles trapezoidal in cross section and arranged in an annular form in combination, and a case 1 having the nib 2 inserted therein. A regular hexagonal die bore 4 is formed by the nib pieces 3 inside thereof. The tool is used mainly for manufacturing bolts or nuts. A blank (not shown) is inserted into the die bore 4 and pressed to a specified shape by the tool around the blank.
However, it is likely that the material of the blank will bite in between the butting ends of the adjacent nib pieces 3, 3 to create a clearance therebetween by being pressed within the die bore 4. The clearance, once produced, permits the blank material to wedge in more readily, enlarging the clearance between the nib pieces 3, 3 at an accelerated rate.
When the clearance between the nib pieces 3, 3 is enlarged, the portion of blank material wedging into the clearance remains on the pressed product as a burr to make the product faulty. The tool then needs to be replaced by a new one.